1. Field of the invention
This invention pertains to a velocity change sensor or accelerometer used in motor vehicles for detecting sudden changes in velocity and for activating in response a passenger restraining device such as an air bag. More particularly, this device includes an element which moves to a preset position in response to a sudden deceleration to touch a pair of contact blades, said device also having a contact retainer for positioning the contact blades in a preselected manner.
2. Description of the Related Art
Studies have been made which indicate that injuries in motor vehicle accidents, especially at high speeds, can be substantially reduced or eliminated by the use of passenger restraint systems. (The term "passenger" is used to cover the driver of a car as well.) These systems include an inflatable balloon usually termed an air bag which normally is stored away in the instrument panel or the steering wheel. When the motor vehicle is subjected to a sudden deceleration, the air bag is inflated and is deployed automatically in a position which cushions the passengers, restrains their movement and prevents contact between them and the automobile interior such as the windshield, the steering wheel. The instrument panel and so on. Of course, a crucial element of all such systems is the velocity change sensor or accelerometer which initiates the inflation and deployment of the air baqs. The motion of the motor vehicle must be carefully and precisely monitored so that the air bags can be deployed very fast, before the passengers suffer any substantial injury.
A velocity change sensor is disclosed in U.S. Pat. No. 4,329,549 assigned to the same company as the present invention. This sensor comprises a tubular housing surrounding a metallic shell, a metal ball and a magnet biasing the ball toward a first end of the shell. At the second end of the shell there are a pair of electrical contact blades. The sensor is positioned in the motor vehicle in an orientation such that when the motor vehicle experiences a deceleration which exceeds a preset level, the ball moves from the first toward the second end, making contact with the two blades. Because the blades and the ball are made of on electrically conducting material, when the ball contacts the blades, an electrical path is established between the two blades. This electrical path is used to initiate a signal for the deployment of the air bags.
Accuracy of the sensor described in U.S. Pat. No. 4,329,549 depends to a large extent on the proper alignment of the two electrical contact blades to insure that they will contacted by the ball simultaneously. When the two blades are not aligned evenly, they are disposed at different distances with respect to an end of the shell. Therefore during the deceleration of the motor vehicle, as the ball moves toward the second end of the shell, it touches one of the blades first. This touch slows the ball down so that precious time is wasted before ball touches the second contact blade to establish the electrical path. However, it is essential that the time between the motor vehicle starts decelerating and the deployment of the air bag be kept at an absolute minimum and constant value. If this delay is too long, the air bag cannot perform its function of protecting the passengers.
The contact blades are typically made of a relatively thin material such copper or a similar alloy which is flexible and easy to bend. Because of these characteristics, it has proven very difficult at times and somewhat time consuming on occasion to manufacture and assemble these blades so that they are aligned evenly so that the ball will contact each blade at the same time. To maintain a timely and predictable reaction time of the sensor, it is also essential that the distance from the ball rest position to the contact point be constant. The contact retainer also achieves this goal. Furthermore, even if they are aligned perfectly initially, these blades are frequently bent out of alignment during the assembly and installation of the sensor.